The present invention relates to a motor and a turbo-molecular pump.
A turbo-molecular pump produces an ultra-high vacuum state and is employed in, for example, semiconductor fabrication related apparatuses (e.g., sputtering apparatuses, chemical vapor deposition (CVD) apparatuses, and etching apparatuses) and measuring apparatuses (e.g., electron microscopes, surface analysis apparatuses, and environment testing apparatuses). A typical turbo-molecular pump includes a motor and a plurality of rotor vanes rotated by the motor. The rotor vanes are rotated to produce a molecular flow and discharge gases. This causes an ultrahigh vacuum state in the interior of the apparatus connected to the turbo-molecular pump.
The motor has a rotary shaft that is rotated at a high speed to produce the ultra-high vacuum state. The bearing that supports the rotary shaft must thus be capable of high speed rotation. A ball bearing, which requires lubricating oil, is not appropriate for such application. This is because the vapor pressure of the lubricating oil, although low, hinders depressurization to the ultra-vacuum state by the turbo-molecular pump. Further, vaporized lubricating oil contaminates vacuum chambers.
Japanese Unexamined Utility Model Publication No. 63-14894 and Japanese Unexamined Patent Publication No. 2-16389 describes a turbo-molecular pump that does not use lubricating oil. This turbo-molecular pump employs non-contact bearings, such as air bearings or magnetic bearings.
A kinetic air bearing is one example of an air bearing. This air bearing has a fixed cylinder and a rotatable cylinder, which is arranged in the fixed cylinder. A bearing area and a seal area are defined on the outer surface of the rotatable cylinder. A plurality of dynamic pressure grooves extend along the bearing area. A predetermined clearance is provided between the outer surface of the rotatable cylinder and the inner surface of the fixed cylinder.
One end of the two cylinders is exposed to a predetermined vacuum atmosphere. Thus, the seal area is located near that end of the rotatable cylinder to prevent gases from moving between a compressed gas layer in the bearing and the vacuum atmosphere. A plurality of helical seal grooves extend along the seal area. An annular groove formed on the outer surface of the rotatable cylinder extends along a boundary area defined between the bearing area and the seal area. An aperture extends through the fixed cylinder at a location opposed to the annular groove. The motor drives and rotates the rotatable cylinder. The rotation causes the air outside the fixed cylinder to pass through the aperture and into the clearance (especially to the region between the bearing area and the opposed area of the fixed cylinder). This forms a pressure gas film, or the compressed gas layer, which radially supports the rotary shaft.
When the rotating speed of the rotatable cylinder is lower than a predetermined value, the rotatable cylinder slides on the fixed cylinder. Ceramics having relatively high anti-wear properties, such as alumina and zirconia, may be used as the material of the fixed and rotatable cylinders.
When designing the turbo-molecular pump, the depressurizing capability of the motor determines the number of vanes and the motor speed. For example, the motor speed is 50,000 rpm to 70,000 rpm in a typical, compact turbo-molecular pump.
The viscous friction produced by air increases the temperature of the air bearing during high speed rotation. The generated heat is transferred rather easily from the outer surface of the fixed cylinder. On the other hand, since the rotatable cylinder is covered by the fixed cylinder, heat cannot be transferred from the rotatable cylinder so easily. This results in a large difference between the temperature of the fixed cylinder and the temperature of the rotatable cylinder. The coefficient of thermal expansion for ceramics, such as alumina and zirconia, is 7 to 8xc3x9710xe2x88x926/xc2x0 C. and thus relatively high. Therefore, in an air bearing made of alumina or zirconia, the temperature difference between the fixed cylinder and the rotatable cylinder causes the dimension change of the fixed cylinder to differ from that of the rotatable cylinder. This varies the size of the clearance. Consequently, the rotatable cylinder may contact the fixed cylinder and obstruct high speed rotation.
A fan is often used to cool the air bearing. The fan is effective for cooling the outer part of the air bearing, or the fixed cylinder, but not for cooling the inner part, or the rotatable cylinder. Hence, the fan further increases the temperature difference between the rotatable cylinder and the fixed cylinder and changes the size of the clearance. There is a demand for a more compact turbo-molecular pump that operates at higher rotating speeds. In such a pump, the size of the clearance must be decreased. Therefore, the effects of heat on the air bearing cannot be ignored.
To increase the speed of the motor, a bearing that has an improved seal and improved performance is necessary. The vibrations of the rotatable cylinder affect the supporting characteristics of the rotatable cylinder. For example, the depth of the dynamic pressure grooves affect the natural frequency of the rotatable cylinder. When the natural frequency (Hz) of the rotatable cylinder and the rotating speed (rps) of the rotatable cylinder are about the same, the possibility of resonance is high. Resonance causes vibrations of the motor. Therefore, the depth of the dynamic pressure grooves is an important factor for obtaining improved bearing characteristics. Further, the depth of the seal grooves affects the seal characteristics. Hence, the depth of the seal grooves is an important factor for obtaining a high degree of vacuum.
It is an object of the present invention to provide a compact motor applicable to high speeds and a compact turbo-molecular pump having a high depressurizing capability.
To achieve the above object, the present invention provides a motor including a rotary shaft and a bearing for radially supporting the rotary shaft. The bearing includes a cylindrical rotary member connected to the rotary shaft, and a cylindrical fixed surface surrounding the rotary member. The fixed surface is spaced from the rotary member by a predetermined distance. The material of the rotary member has a coefficient of thermal expansion that is smaller than that of the material of the fixed surface.
Another aspect of the present invention provides a motor including a rotary shaft and a bearing for radially supporting the rotary shaft, wherein the bearing includes a cylindrical rotary member connected to the rotary shaft and a cylindrical fixed surface surrounding the rotary member. The fixed surface is spaced from the rotary member by a predetermined distance. The rotary member is made of a material having a coefficient of thermal expansion that is 5xc3x9710xe2x88x926/xc2x0 C. or less.
A further aspect of the present invention provides a turbo-molecular pump including a housing, a stator vane attached to the housing, a rotor vane rotated relative to the stator vane, and a motor for driving the rotor vane. The motor includes a rotary shaft and a bearing for radially supporting the rotary shaft. The bearing includes a cylindrical rotary member connected to the rotary shaft and a cylindrical fixed surface surrounding the rotary member. The fixed surface is spaced from the rotary member by a predetermined distance. The material of the rotary member has a coefficient of thermal expansion that is smaller than that of the material of the fixed surface.
A further aspect of the present invention provides a turbo-molecular pump including a housing, a stator vane attached to the housing, a rotor vane rotated relative to the stator vane, and a motor for driving the rotor vane. The motor includes a rotary shaft and a bearing for radially supporting the rotary shaft. The bearing includes a cylindrical rotary member connected to the rotary shaft and a cylindrical fixed surface surrounding the rotary member. The fixed surface is spaced from the rotary member by a predetermined distance. The rotary member is made of a material having a coefficient of thermal expansion that is 5xc3x9710xe2x88x926/xc2x0 C. or less.
A further aspect of the present invention provides a motor including a rotary shaft and a bearing for radially supporting the rotary shaft. The bearing includes a cylindrical rotary member connected to the rotary shaft and a cylindrical fixed surface surrounding the rotary member. The fixed surface is spaced from the rotary member by a predetermined distance. At least one of the rotary member and the fixed surface has a dynamic pressure groove formed on a predetermined first area defined on a surface opposing the other of the rotary member and the fixed surface. At least one of the rotary member and the fixed surface has a seal groove formed on a predetermined second area defined on a surface opposing the other one of the rotary member and the fixed surface. The seal groove is formed deeper than the dynamic pressure groove.
A further aspect of the present invention provides a turbo-molecular pump including a housing, a stator vane attached to the housing, a rotor vane rotated relative to the stator vane, and a motor for driving the rotor vane. The motor includes a rotary shaft and a bearing for radially supporting the rotary shaft. The bearing includes a cylindrical rotary member connected to the rotary shaft and a cylindrical fixed surface surrounding the rotary member. The fixed surface is spaced from the rotary member by a predetermined distance. At least one of the rotary member and the fixed surface has a dynamic pressure groove defined on a surface opposing the other of the rotary member and the fixed surface. At least one of the rotary member and the fixed surface has a first seal groove formed on a surface opposing the other of the rotary member and the fixed surface. The first seal groove is formed deeper than the dynamic pressure groove.